Zoom lens system

Abstract

A zoom lens system includes a negative first lens group, a positive second lens group, and a positive third lens group, in this order from the object. Upon zooming from the short focal length extremity to the long focal length extremity, the distance between the negative first lens group and the positive second lens group decreases, and the distance between the positive second lens group and the positive third lens group increases. The negative first lens group includes a negative first lens element, a positive or negative lens element having a weaker refractive power, and a positive third lens element, in this order from the object. The zoom lens system satisfied the following condition: −0.25< f 1/ fp <0.25( f 1<0)  (1) wherein f1 designates the focal length of the negative first lens group; and fp designates the focal length of the positive second lens element.

1. A zoom lens system comprises a negative first lens group, a positive second lens group, and a positive third lens group, in this order from an object,
 wherein upon zooming from the short focal length extremity to the long focal length extremity, the distance between said negative first lens group and said positive second lens group decreases, and the distance between said positive second lens group and said positive third lens group increases; 
 wherein said negative first lens group comprises a negative first lens element, a positive or negative second lens element having a weaker refractive power, and a positive third lens element, in this order from the object; 
 wherein said zoom lens system satisfied the following condition:
   −0.25 <f 1 /fp< 0.25( f 1<0) 
 
 wherein 
 f1 designates the focal length of said negative first lens group; and 
 fp designates the focal length of said second lens element; 
 wherein said positive third lens group is arranged to function as a focusing lens group, and comprises a biconvex plastic lens element having at least one aspherical surface; and 
 wherein said zoom lens system satisfies the following conditions:
   3.1< f 2/ rs< 3.6 
   0.6< f 2/ f 3<1.0 
 
 wherein 
 f2 designates the focal length of said positive second lens group; 
 rs designates a radius of curvature of the most image-side surface of said positive second lens group; and 
 f3 designates the focal length of said positive third lens group. 
 
   
   
     2. The zoom lens system according to  claim 1 , wherein said second lens element of said negative first lens group satisfies the following condition:
   −0.3<( ra−rb )/( ra+rb )<0.3 
 wherein 
 ra designates a radius of curvature of the object-side surface of said second lens element of said negative first lens group; and 
 rb designates a radius of curvature of the image-side surface of said second lens element of said negative first lens group. 
 
   
   
     3. The zoom lens system according to  claim 1 , wherein said second lens element of said negative first lens group comprises at least one aspherical surface, and said second lens element satisfies the following condition:
   n2<1.55 
 wherein 
 n2 designates the refractive index of the d-line of said second lens element. 
 
   
   
     4. The zoom lens system according to  claim 1 , wherein upon zooming from the short focal length extremity to the long focal length extremity, said positive second lens group is monotonically moved toward the object, and satisfies the following condition:
   3.2< m 2 t/m 2 w< 3.8 
 wherein 
 m2t designates a magnification of said positive second lens group, at the long focal length extremity, when an object at infinity is in an in-focus state; and 
 m2w designates a magnification of said positive second lens group, at the short focal length extremity, when an object at infinity is in an in-focus state. 
 
   
   
     5. The zoom lens system according to  claim 1 , wherein upon zooming from the short focal length extremity to the long focal length extremity, said positive third lens group is monotonically moved toward the image, and satisfies the following condition:
   1.05< m 3 t/m 3 w< 1.3 
 wherein 
 m3t designates a magnification of said positive third lens group, at the long focal length extremity, when an object at infinity is in an in-focus state; and 
 m3w designates a magnification of said positive third lens group, at the short focal length extremity, when an object at infinity is in an in-focus state. 
 
   
   
     6. The zoom lens system according to  claim 1 , wherein said negative first lens group satisfies the following condition:
   0.2< t 1/| f 1|<0.4 
 wherein 
 t1 designates a distance from the most object-side surface of said negative first lens group to the most image-side surface thereof; and 
 f1 designates the focal length of said negative first lens group. 
 
   
   
     7. The zoom lens system according to  claim 1 , wherein the most object-side positive lens element of said positive second lens group is provided, on each surface, with an aspherical surface which is formed so that curvature becomes smaller toward the periphery, compared with a paraxial spherical surface. 
   
   
     8. The zoom lens system according to  claim 1 , wherein the most object-side lens element of said positive second lens group comprises a biconvex positive lens element, and satisfies the following condition:
   65<νp 
 wherein 
 νp designates the Abbe number of said biconvex positive lens element. 
 
   
   
     9. The zoom lens system according to  claim 1 , wherein said second lens group comprises a positive lens element, a positive lens element, and a negative lens element, in this order from the object. 
   
   
     10. A zoom lens system comprises a negative first lens group, a positive second lens group, and a positive third lens group, in this order from an object,
 wherein upon zooming from the short focal length extremity to the long focal length extremity, the distance between said negative first lens group and said positive second lens group decreases, and the distance between said positive second lens group and said positive third lens group increases; 
 wherein said negative first lens group comprises a negative first lens element, a positive or negative second lens element having a weaker refractive power, and a positive third lens element, in this order from the object; 
 wherein said zoom lens system satisfied the following condition:
   −0.25< f 1 /fp< 0.25( f 1<0) 
 
 wherein 
 f1 designates the focal length of said negative first lens group; and 
 fp designates the focal length of said second lens element; 
 wherein upon zooming from the short focal length extremity to the long focal length extremity, said positive second lens group is monotonically moved toward the object, and satisfies the following condition:
   3.2< m 2 t/m 2 w< 3.8 
 
 wherein 
 m2t designates a magnification of said positive second lens group, at the long focal length extremity, when an object at infinity is in an in-focus state; and 
 m2w designates a magnification of said positive second lens group, at the short focal length extremity, when an object at infinity is in an in-focus state. 
 
   
   
     11. The zoom lens system according to  claim 10 , wherein said second lens element of said negative first lens group satisfies the following condition:
   −0.3<( ra−rb )/( ra+rb )<0.3 
 wherein 
 ra designates a radius of curvature of the object-side surface of said second lens element of said negative first lens group; and 
 rb designates a radius of curvature of the image-side surface of said second lens element of said negative first lens group. 
 
   
   
     12. The zoom lens system according to  claim 10 , wherein said second lens element of said negative first lens group comprises at least one aspherical surface, and said second lens element satisfies the following condition:
   n2<1.55 
 wherein 
 n2 designates the refractive index of the d-line of said second lens element. 
 
   
   
     13. The zoom lens system according to  claim 10 , wherein upon zooming from the short focal length extremity to the long focal length extremity, said positive third lens group is monotonically moved toward the image, and satisfies the following condition:
   1.05< m 3 t/m 3 w< 1.3 
 wherein 
 m3t designates a magnification of said positive third lens group, at the long focal length extremity, when an object at infinity is in an in-focus state; and 
 m3w designates a magnification of said positive third lens group, at the short focal length extremity, when an object at infinity is in an in-focus state. 
 
   
   
     14. The zoom lens system according to  claim 10 , wherein said negative first lens group satisfies the following condition:
   0.2< t 1/| f 1|<0.4 
 wherein 
 t1 designates a distance from the most object-side surface of said negative first lens group to the most image-side surface thereof; and 
 f1 designates the focal length of said negative first lens group. 
 
   
   
     15. The zoom lens system according to  claim 10 , wherein the most object-side positive lens element of said positive second lens group is provided, on each surface, with an aspherical surface which is formed so that curvature becomes smaller toward the periphery, compared with a paraxial spherical surface. 
   
   
     16. The zoom lens system according to  claim 10 , wherein the most object-side lens element of said positive second lens group comprises a biconvex positive lens element, and satisfies the following condition:
   65<νp 
 wherein 
 νp designates the Abbe number of said biconvex positive lens element. 
 
   
   
     17. The zoom lens system according to  claim 10 , wherein said second lens group comprises a positive lens element, a positive lens element, and a negative lens element, in this order from the object.